Metatrader 4 is supposed to work on all Windows platforms.
But on recent releases as Vista SP2 and Windows 7 it seem not working 100%.
This is happening especially when Expert Advisors work with external DLL.
On those systems some dll are loaded at a wrong memory address or Metatrader overlap some memory addresses allocated for specific dll.

SignalCopy tools was tested on all Windows platforms starting with Windows 2000 on 32 bits and  64 bits versions.
All tools was also tested on Linux and proved to work.

Most problems were reported on Vista and Windows 7 and we had to rewrite dll files to workaround problems with those OS.
Please note, is not fault of our DLL or experts advisors, is compatibility of Metatrader with Vista ans Windows 7.

http://www.forex-tsd.com/non-related-discussions/22735-windows-7-metatrader-4-a.html
http://www.eggheadcafe.com/software/aspnet/30561342/meta-trader-mt4-stopped.aspx
http://www.metatrader4.com/forum/13361

and many more on Google.

So, what is recommended OS?
According to tests and users feedback, most reliable system is Windows XP (SP2 or SP3) and worst system is Vista.

On Windows 7 is recommended to use XP mode

Below are listed some info and highlights for most common operating systems (XP, Vista and Windows 7)
MetaTrader ® 4
Information and trading platform developed by MetaQuotes Software Corp. Intended for brokerage services[1] in the markets of Forex, CFD and Futures.
This set of full-cycle, ie for brokerage services in the presence of MetaTrader 4 does not require any additional software.
It works on Windows, other operating systems are not supported.


http://en.wikipedia.org/wiki/Windows_XP
As of the end of December 2009, Windows XP is the most widely used operating system in the world with a 61.6% market share, having peaked at 76.1% in January 2007.

System requirements for Windows XP Home and Professional editions as follows:
                       Minimum            Recommended
Processor     233 MHz1          300 MHz or higher
Memory         64 MB RAM2     128 MB RAM or higher
Video adapter and monitor     Super VGA (800 x 600) or higher resolution
Hard drive disk free space     1.5 GB or higher
(additional 1.8 GB in Service Pack 2 and additional 900 MB in Service Pack 3)

Support lifecycle
On April 8, 2014, all Windows XP support, including security updates and security-related hotfixes will be terminated.

http://en.wikipedia.org/wiki/Windows_Vista
At the release of Windows 7 (October 2009), Windows Vista (with approximately 400 million Internet users) was the second most widely used operating system on the Internet with an approximately 23.6% market share, the most widely used being Windows XP with an approximately 60% market share.
Windows Vista ships in eight editions.

Windows Vista system requirements    

                                  Vista Capable     Vista Premium Ready
Processor                 800 MHz[60]     1 GHz
Memory                     512 MB              1 GB
Graphics card     DirectX 9.0 capable     DirectX 9.0 capable and WDDM 1.0 driver support
Graphics memory   32 MB                 128 MB
HDD capacity          20 GB                  40 GB
HDD free space     15 GB

http://en.wikipedia.org/wiki/Criticism_of_Windows_Vista
Criticism of Windows Vista
User Account Control
Driver signing requirement
Flaws in memory protection features

There were reports of Vista users "downgrading" their operating systems, as well as reports of businesses planning to skip Vista. A study conducted by ChangeWave in March 2008 showed that the percentage of corporate users who were "very satisfied" with Vista was dramatically lower than other operating systems, with Vista at 8%, compared to the 40% who said they were "very satisfied" with Windows XP.

http://en.wikipedia.org/wiki/Windows_7
Windows 7 was released to manufacturing on July 22, 2009, and reached general retail availability on October 22, 2009, less than three years after the release of its predecessor, Windows Vista.
Windows 7 is available in six different editions, but only the Home Premium, Professional, and Ultimate editions are available for retail sale to consumers in most countries

Minimum hardware requirements for Windows 7 Architecture   
                              32-bit                                              64-bit
Processor           1 GHz 32-bit processor                1 GHz 64-bit processor
Memory (RAM)   1 GB of RAM                                  2 GB of RAM
Graphics Card     DirectX 9 graphics processor with WDDM driver model 1.0 (For Aero)
HDD free space  16 GB of available disk space     20 GB of available disk space

Additional requirements to use certain features:

    * Windows XP Mode (available on Professional, Enterprise and Ultimate), requires an additional 1 GB of RAM, an additional 15 GB of available hard disk space and a processor capable of hardware virtualization with Intel VT or AMD-V enabled.
    * Windows Media Center (included with Home Premium, Professional, Enterprise, Ultimate), requires a TV-tuner to receive and record TV.

Windows 7 Beta beat both Windows XP and Vista in several key areas; including boot and shutdown time and working with files, such as loading documents.
Other areas did not beat XP; including PC Pro benchmarks for typical office activities and video editing, which remain identical to Vista and slower than XP.
Reviews of Windows 7 were mostly positive, praising its usability when compared to its predecessor, Windows Vista


Comparison of Microsoft Windows minimum hardware requirements (for 32-bit versions).
Windows version Processor     Memory     Hard disk
Windows 95          25 MHz         4 MB          ~50 MB
Windows 98          66 MHz         16 MB        ~200 MB
Windows 2000     133 MHz       32 MB        650 MB
Windows XP         233 MHz       64 MB       1.5 GB
Windows Vista     800 MHz       512 MB     15 GB
Windows 7           1 GHz             1 GB          16 GB